GB2113010A - Electric immersion heater - Google Patents

Abstract

An electric immersion heater for an electric kettle or like container includes a thermally-sensitive control 1 mounted to the head 2 thereof. The control 1 includes electrical switch means operable in use to interrupt the power supply to the heater and a snap acting bimetallic actuator 56 which is exposed to steam resulting from liquid in an associated container boiling, the steam being passed from the inside of the container via a tube 6 carried by the heater head 1. Access for steam to the actuator 56 is provided by a passage 60 formed in a body portion 10 of the control, the end of the steam tube 6 being sealed to the passage 60 by means of a resilient sleeve 61 one end of which embraces the tube outlet and the other end of which is disposed in the passage 60. <IMAGE>

Description

SPECIFICATION Electric immersion heater This invention relates to electric immersion heaters for containers such as electric kettles, jugs, coffee percolators, pans, urns, laboratory equipment and the like. Such immersion heaters commonly comprise a heating element carried by a head which is adapted to be secured in a watertight manner in or adjacent an opening in the container wall. The heating elements of such heaters terminate in so-called cold leads which may be connected to a source of electrical power.

Such immersion heaters are referred to hereinafter as "of the kind described".

Such immersion heaters often include thermally sensitive electrical controls having electrical switch means for interrupting the power supply to the element upon liquid within the container boiling. In one common arrangement, the heater carried a tube for the egress of steam or vapour from the interior of the container which tube extends from above water level within the container to a thermally sensitive control mounted externally to the container. An example of such an arrangement is disclosed in British Patent No. 1,316,436 wherein the steam tube passes through the head of the immersion heater and is secured thereto. A moulded body portion of the control is secured externally to the head and is formed with an aperture therein which receives the end of the steam tube.The tube is sealed within the aperture by means of a resilient "0" ring which prevents leakage of steam to electrical components of the assembly. The aperture communicates with a channel which, during boiling, transmits steam or vapour to a thermally responsive actuator for the switch means to interrupt the power supply to the element. A similar arrangement is described in British Patent Specification No. 2042269A wherein the end of the steam pipe is again sealed in an aperture in a body portion of the control by means of an "O" ring. An actuator for the electrical switch means is in this case positioned within the control immediately adjacent the end of the steam tube.

It is clear that with these arrangements the diameter of the steam tube must be less than the diameter of the receiving aperture therefor in the body portion of the control and, since the space available for such an aperture is relatively limited, the maximum possible diameter for the steam tube is restricted. However, with such immersion heaters it is desirable that the diameter of the steam tube be as large as possible so as to avoid blocking of the tube due to steam or vapour condensing therein; this problem can be particularly significant if the heater is fitted to a container not originally adapted for use with a steam tube since, with such containers, the steam pressure within the container during boiling might be insufficient adequately to drive steam or vapour through a narrow tube.The probiem of blocking of the steam tube is recognised in British Patent Specification No. 1384527 in which the inlet end of the steam tube is disposed in an enclosure or steam trap which extends below water level and constitutes in effect a boiler which forces steam or vapour through the tube at increased pressure.

However, this modification leads to a substantial increase in the cost of the heater.

A further drawback associated with known heater assemblies of this type is that the steam tube must be secured to the head at a precise position within relatively fine tolerances so as to ensure that the tube aligns correctly with the receiving aperture or passage therefor formed within the body portion. This causes manufacturing difficulties since the steam tube is generally brazed to the element head and brazing is not inherently a precision operation.

According to the invention there is provided an electric immersion heater of the kind described including a tube for the egress of steam or vapour from the interior of an associated container in use of the assembly and an electrical control mounted to or adjacent the head of said heater including electrical switch means operable upon steam or vapour passing through said tube to cut off the power supply to the heating element when the liquid within the container boils, said tube having an inlet communicating with the interior of the container above the level of the liquid therein and an outlet for delivering steam or vapour to a thermally responsive actuating means for said switch means, there being an aperture or passage formed in a body portion of the control providing access for steam or vapour to said actuating means, one end of a resilient sleeve embracing said tube outlet and the other end of said sleeve being disposed within said aperture or passage.

The use of a resilient sleeve is convenient from the manufacturer's point of view in that, unlike previous arrangements, the steam tube need not be precisely positioned within the head relative to the body portion aperture or passage since the tube will accommodate slight misaiignment therebetween. Furthermore, since the end of the tube is not disposed within the aperture or passage, the diameter of the tube may be larger than the diameter of the aperture or passage in which case the tube is tapered to accommodate the difference in diameters. Thus, a larger bore of steam tube may be used as compared with known arrangements and the aforesaid problems concerning blocking of the tube may be reduced.

The resilient sleeve is preferably formed from an elastic heat resistant material such as silicone rubber. In this case the end of the sleeve which embraces the tube may simply be stretched over the tube outlet, while the other end of the sleeve is pushed into the body portion aperture and shapes itself so as to fit snugly therein. Moreover, where the steam tube is of greater diameter than the aperture or passage an elastic sleeve need not be preformed to accommodate the difference in diameters since it will readily adopt the required tapered shape.

The actuating means may take any appropriate form or may conveniently comprise a snap-acting bimetallic actuator such as that described in British Patent Specification No. 1 542252.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is an exploded perspective view of an electrical control for an immersion heater in accordance with the invention taken from the side of the control which in use is secured to the immersion heater head.

Figure 2 is an exploded perspective view of the control illustrated in Figure 1 taken from the opposite side of the control and showing the immersion heater head.

Figure 3 is a perspective view similar to Figure 2 with certain parts of the control removed.

Figure 4 is a cross-sectional view taken from one side of the control of Figures 1 to 3 mounted to the head of an immersion heater and in a deactivated position.

Figure 5 is a view similar to Figure 4 but showing the overcentre mechanism in the operational position.

Figure 6 is an elevational view, partly broken away, of the end of the control adapted to be secured to the immersion heater head.

Figure 7 is a cross-sectional view taken along line 7-7 in Figure 6 showing contacts of the switch means open.

Figure 8 is a view similar to Figure 7 but showing the switch contacts closed.

Figure 9 is a view similar to Figures 7 and 8 but showing one contact open and one closed.

Figure 10 is a plan view of the overcentre spring for the control illustrated in Figures 1 to 9.

Referring firstly to Figures 1 to 5, a thermallysensitive control 1 is mounted on the head 2 of an electric immersion heater. The cup-like head 2 is retained in a water-tight manner in an opening formed in the wall 4 of an associated container by means of an externally threaded retaining ring 3 which urges a peripheral flange 9 of the head 2 against the inside of the wall 4 compressing a resilient O-ring seal 5 therebetween as shown in Figure 4. The immersion heater head 2 carries a tube 6 for the egress of steam from the interior of the container which tube is brazed to the head and extends, in use, above water level within the container. The immersion heater comprises two cold leads 7 and a hot return 8 which passes underneath the cold leads 7 and is brazed to the head 2 so as to be in good thermal contact therewith.The low position of the hot return 8 enables the container to boil small quantities of liquid if desired.

The control 1 comprises a body portion 10 of thermo-plastic material, mounted snugly within the head 2 by means of three bolts 11 which engage, threaded female studs 12 brazed to the inside of the head, and a cover member 13 which is secured to the body portion 10 by means of a single central fixing bolt 14. The body portion 10 includes a horizontal shelf member 1 5 which lies close to the central longitudinal axial plane of the control 1 and defines at the side of the body portion remote from the head, upper and lower segmental volumes of the control lying respectively above and below the central horizontal axial plane of the control.Three terminal pins 1 6 are secured to and extend through the body portion 10 and include a live pin, a neutral pin and an earth pin for connection to a female socket connector to supply mains electrical power to the heating element. The end portions of the pins 16, which extend parallel to and are disposed below the shelf member 15, are arranged in a triangular relationship with the central or earth pin lying below the live and neutral pins. The cover member 1 5 defines a shroud 1 7 which surrounds the pins 1 6 and is shaped so as to snugly receive a standard socket connector.

The live and neutral pins are each connected to a respective cold lead 7 via a pair of breakable contacts, each pair including a movable contact 1 8 and a stationary contact 19 mounted directly to a respective cold lead 7. The stationary contacts 1 9 may be mounted to the cold leads 7 by means of cylindrical spring clips to which the contacts 19 are secured and which are simply pushed over the ends of the cold leads. The movable contacts 1 8 are mounted towards the free ends 22 of respective generally 'U'-shaped leaf springs 20 formed from resilient metallic strip material.The leaf springs 20 extend upwardly adjacent the side of the body portion 10 facing the head from the bases of the respective live and neutral terminal pins and are secured thereto by means of apertured base portions 21 of the leaf springs 20 which surround the base portions of the pins. During normal operation of the heater, the leaf springs 20 urge the movable contacts 18 against the respective stationary contacts 1 9 and electrical power may be supplied from the live and neutral terminal pins to the element. The earth pin.

by virtue of its disposition towards the outer periphery of the control, is convenientiy electrically connected with the head 2 by means of a metallic spring clip 29 which is secured to the base of the pin and directly engages the inside of the head 2.

A generally 'U' shaped link member 23 of moulded plastics material lies in a horizontal plane of the control slightly below the switch contacts 18, 19 and includes a transverse arm 24 disposed between the body portion 10 and the head 2, beneath the contacts, and two spaced longitudinal arms 25. The longitudinal arms 25 extend through the body portion 10 and are slidably mounted in axially directed channels 26 formed on either side of the body portion 10 close to the outer periphery thereof. The transverse arm 24 of the link member 23 includes two stepped abutments 27 at either end thereof adjacent the longitudinal arms 25 which abutments 27 are adapted to engage respective free ends 22 of the leaf springs 20 whereby movement of the link member 23 in a direction away from the head 2 causes the free ends 22 of the springs 20 and the respective movable contacts 1 8 to be displaced in a direction away from the head 2 thereby opening the contacts and interrupting the power supply to the heater.

In addition, two vertical abutment arms 90 extend upwardly from the link member transverse arm 24 which are provided with abutment surfaces 91 adapted for engagement with the uppermost portions of the respective leaf springs 20. Upon displacement of the link member 24 away from the head, the abutment surfaces 91 engage the leaf springs slightly after engagement thereof by the abutments 27 and urge the contacts further apart.

A first thermally responsive actuating means comprises a snap-action bimetallic actuator 28 of the sort described in British Patent Specification No. 1 ,542,252. The bimetallic actuator 28 is mounted on a stainless steel bridge member 30 as described in more detail below and overlies the bases of the terminal pins 1 6. A lower peripheral portion 31 of the actuator 28 is held in good thermal contact with wedge-shaped protrusion 43 formed on the inside of the head 2 towards the bottom thereof adjacent to the point where the hot return 8 of the element is welded thereto, and an upper bridge portion 32 of the actuator 28 is adapted for engagement with a central abutment surface 33 formed on the 'U'-shaped link member 23.The actuator 28 is calibrated with a nominal operating temperature of 1400C so as to undergo snap action when the element overheats. Thus, when the element overheats after for example having been switched on dry, the actuator 28 reverses its curvature by snap-action whereupon the bridge portion 32 engages and moves rightwardly, i.e. away from the head, the link member 23 which in turn pushes the movable contacts 1 8 away from the stationary contacts 1 9 as shown in figure 7 so as to cut off the power supply to the heating element.

An important safety feature of the illustrated control arises from the fact that the 'U'-shaped link member 23 in addition to being slidable in a substantially axial direction is pivotable to a limited extent in its horizontal plane about a vertical axis. Such pivotal movement is accommodated by the outer edges of the longitudinal arms 25 of the link member 23 being contoured within the channels 26 so as to define curved surfaces 34 which abut the outer wall of respective channels 26 therebeing a lateral clearance between the major part of the edges of the arms 25 and the channels 26.Thus, in the event of either pair of contacts 18, 19 becoming welded together after an extended period of use, upon operation of the actuator 28 engagement of the bridge portion 32 thereof with the abutment surface 33 of the link member 23 will cause the link member 23 to pivot about the welded together pair of contacts and open the remaining pair as shown in Figure 9 thereby de-energising the heater.

The stainless steel resilient bridge member 30 includes a pair of spaced lateral flanges 35 projecting from each side thereof which fit snugly within respective generally rectangular apertures 36 formed at either side of the body portion 10 and through which the lowermost two of the body portion mounting bolts 11 extend. As the bolts 11 are tightened, the flanges 36 bear directly against the respective female studs 12 of the head and are firmly clamped between the heads of the respective bolts 11 and the studs 1 2 within the apertures 36.

The bimetallic actuator 28 is mounted to the bridge member 30 by means of its tongue 37 which passes between two folded-over ears 38 of the bridge member 30 and over a folded-up ramp 39 thereof which enters an aperture in the tongue 37. The portion 41 of the bridge member 30 which carries the bimetallic actuator 28 is longitudinally offset from the side arms 42 of the bridge member so as to provide necessary clearance between the periphery of the actuator and the bridge member for the actuator to reverse its curvature by snap-action and to permit the necessary movement of the bridge portion 32 of the actuator. In addition, as seen most clearly in Figure 4, the portion 41 which carries the ears 38 and the ramp 39 is at an acute angle to the vertical wall of the head 2 so as to ensure that lower peripheral portion 31 of the actuator 28 is in intimate contact with the protrusion 43.

An overcentre spring mechanism 44 is disposed above the shelf member 1 5 on the side of the body portion remote from the element head i.e. above the axial horizontal plane of the control.

The mechanism 44 includes a lever member 45 having a generally semi-cylindrical profile so as to fit snugly within the upper segmental space of the control. The lever member 45 is pivotably mounted to the body portion by means of two knife edges 46 formed integrally with the member 45 which engage in respective pivot notches 51 formed respectively in spaced arms 47 extending from the body portion 10 in a direction parallel to the shelf member 1 5. An overcentre spring 48 extends between notches 49 formed at the free ends of the arms 47 and notches 50 formed in the lever member 45 in the end thereof remote from the knife edges 46 and biases the knife edges 46 into engagement with the pivot notches 51. As illustrated in Figure 10, the wire spring 48 is a combined double spring and includes two Ushaped portions 80, 81 disposed in back-to-back relation.Each U-shaped portion interconnects a pair of generally parallel arms 82, 83 one of which arm 83 is common to both U-shaped portions. The lever member 45 is pivotable between a first stable position as shown in Figure 5 wherein a lower surface of the lever member 45 engages stops 53 formed on the free ends of the arms 47 and a second stable position shown in Figure 4. In moving from the first stable position to the second, the lever member 45 passes through an unstable dead centre position in which the spring notches 49, 50 and the knife edges 46 all lie in a common horizontal plane.

A bimetallic actuator 56 similar to the actuator 28 but having a nominal operating temperature of 800C is located below the lever member 45 and lies at an acute angle to the shelf member 15 between the arms 47 which engage the outer periphery of the actuator 56 and serve to locate the actuator laterally. The base of the actuator 56 rests on support members 57 of the body portion and is retained longitudinally by a projection 58 at the end of the shelf member which abuts the bridge portion of the actuator 56. The actuator has a peripheral cut-out 100 which engages a projection to prevent rotation of the actuator.

During assembly, the actuator is simply dropped into position and is later secured in place when the cover member 13 is mounted by means of flanges 59 thereof which extend over the bridge portion of the actuator 56 thereby clamping it to the shelf member 15.

Steam or vapour resulting from liquid within the container boiling is transmitted to the actuator 56 via an aperture 60 formed in the body portion 10. The steam tube 6 is sealed to the aperture 60 by means of a silicone rubber sleeve 61 one end of which is stretched over the end of the tube 6 and the other end of which is pushed into the aperture 60. The upper wall of the aperture 60 is upwardly tapered at the side adjacent the head. The lower wall however has no such taper as it has been found that this can result in condensed liquid collecting at adjacent the tube outlet which in turn can lead to blocking of the tube. It will be seen that the tapered sleeve 61 permits use of a wider bore of steam tube as compared with known arrangements wherein the tube has been of smaller radius than the aperture and has been sealed within the aperture by means of O-ring seals.In the preferred embodiment, a steam pipe having an external diameter of 8 mm is used whereas with known arrangements the diameter is typically restricted to 5 mm. Furthermore with known arrangements the positioning of the steam tube relative to the head has been very critical in order to ensure that the end of the tube precisely aligns with the aperture. The use of a flexible sleeve avoids the fine tolerances otherwise involved and thus represents a considerable advantage to the heater manufacture since the steam tube is generally brazed to the head and brazing is not inherently a precision operation.

Upon liquid within the container boiling, steam or vapour impinges on the steam sensitive bimetallic actuator 56 which then reverses its curvature by snap-action so that its tongue 62 engages and bears upwardly against a post 63 extending downwardly from the lever member 45 thus tripping the lever member 45 from its first stable position to its second stable position. The lever member 45 includes two downwardly projecting legs 64 which extend either side of the shelf member 1 5 and terminate within notches 65 formed in the longitudinal arms 25 of the 'U'-shaped link member 23.A strengthening bridge 66 extends between the legs 64 and passes between the body portion arms 47 and shelf member 1 5. Thus, upon the lever member 45 being tripped from its first stable position to its second stable position the legs 64 thereof engage the ends of the notches 65 and pull the link member 23 rightwardly away from the element head and open the switch contacts as described above. The effect is that when the liquid within the container boils the actuator 56 trips the lever member 45 from its first to its second position thereby opening the switch contacts and deenergising the heating element.

As described above, the steam or vapour is shielded from the electrical components on the side of the body portion facing the head by means of the resilient sleeve 61. On the lower side of the body portion, the terminal pins 1 6 are shielded from steam and from condensed liquid by the shelf member 15 and by the cover member shroud 17.

Liquid condensing on the shelf member 1 5 runs off the member at each side into a peripheral passage defined between the shroud 17 and the outer wall of the cover member 13. An aperture 67 is provided at the base of the cover member to allow condensed liquid within the peripheral passage to drain out of the control. The shroud forming portion of the cover member cooperates with the body portion to form a capillary seal 101 therebetween to prevent liquid seeping into contact with the terminal pins.

A knob 68 is pivotably mounted above the lever member 45 for manually resetting the control after the overcentre mechanism has been tripped as a result of liquid within the container boiling.

The knob 68 is mounted by means of a flange 70 thereof which is held between a shoulder 69 projecting from the body portion and part of the cover member. When the knob 68 is pressed downwardly, the lever member 45 is returned to its first position thus allowing the movable contacts 18 to move into contact with the respective stationary contacts 1 9 and the electrical power supply to the element can be resumed. Upon movement of the link member in response to operation of the switch-on-dry actuator 28, the lever member is tripped slightly after the contacts are opened. Therefore, if an attempt is made to energise the heater with the container empty by manually holding the knob in the closed position, the actuator 28 will cycle causing the contacts to repeatedly open and close thus preventing serious overheating of the element.

The embodiment of the control illustrated herein is particularly suitable for sale readymounted to an immersion heater carrying a steam tube as a replacement element for home-fitting.

Claims (4)

1. An electric immersion heater of the kind described including a tube for the egress of steam or vapour from the interior of an associated container in use of the assembly and an electrical control mounted to or adjacent the head of said heater including electrical switch means operable upon steam or vapour passing through said tube to cut off the power supply to the heating element when the liquid within the container boils, said tube having an inlet communicating with the interior of the container above the level of the liquid therein and an outlet for delivering steam or vapour to a thermally responsive actuating means for said switch means, there being an aperture or passage formed in a body portion of the control providing access for steam or vapour to said actuating means, one end of a resilient sleeve embracing said tube outlet and the other end of said sleeve being disposed within said aperture or passage.

2. An electric immersion heater as claimed in claim 1 wherein the diameter of the steam tube is greater than the diameter of said aperture or passage.

3. An electric immersion heater as claimed in claim 1 or 2 wherein said sleeve is formed from silicone rubber.

4. An electric immersion heater substantially as herein described with reference to the accompanying drawings.